(MIC) Polarizing Microscope Lesson

Polarizing Microscope

While most microscopes use basic physics optical principles such as reflection and refraction to create magnified images, the polarizing microscope functions in a more complex manner. Before diving into an explanation of how polarizing microscopes function, it is important to review the basic physics concepts of refraction and index of refraction.

View the video on refraction below before moving forward in the content.

A polarizing microscope is structurally similar to a compound light microscope, however it has a "polarizer" in the condenser which can be rotated to analyze the sample. The polarizer decomposes a ray of light into two rays with different velocities in a process known as birefringence or double refraction. Birefringence of an object is measured by measuring the difference between the two indices of refraction of the components of the material. To visualize what a polarizer does, imagine a white light ray which emits light in wavelengths of visible light in all directions. A polarizer is like a filter that only allows light in a particular direction through and blocks all other directions of the light ray. Polarized light microscopy enhances contrast between the material and the background which reveals specific information about the structure and composition of the substance being analyzed.

Polarizing Light Diagram

polarizing light diagram: WHITE LIGHT POLARIZING FILTER DIRECTION OF PROPAGATION POLARIZED LIGHT PLANE POLARIZED LIGHT

Birefringence Diagram

The polarizing microscope often highlights structural differences that can't be seen in regular lighted conditions; these differences can be used to identify the material being tested. It is used to study birefringent materials from crime scenes such as crystalline solids, some illicit drugs, and certain fibers.

Below are examples of polarized microscope images: 

The one on the left is volcanic sand fragment comparison under light microscope (on top) and with a crossed polarized light (on bottom).  The middle is a polarized microscope image of citric acid, and third is a polarized microscope image of a drug known as DDA, which is used to treat HIV.

POLARIZED MICROSCOPE IMAGE OF CITRIC ACID VOLCANIC SAND FRAGMENT COMPARISON UNDER LIGHT MICROSCOPE (ABOVE) AND WITH A CROSSED POLARIZED LIGHT (BELOW) POLARIZED MICROSCOPE IMAGE OF A DRUG KNOWN AS DDA, WHICH IS USED TO TREAT HIV

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